1. Field of the Invention
This invention relates generally to amplifiers, and in particular, but without limitation thereto, to high power, low distortion, high efficiency amplifiers for use in powerful (multiple megawatt) radio transmitters.
2. Description of the Related Art
The construction of power amplifiers for transmitters has followed two basic approaches. The first of these has been an all-tube design using tubes in a class A, class B, or class C type amplifier circuit topology. A primary disadvantage of this approach is that the mechanism for amplification is through dissipation. Thus the theoretical maximum for efficiency is 50 percent or 70-some percent depending upon circuit design. In extremely large transmitters (the United States Navy has 2,000,000 watt (2 MW) transmitters) energy consumption is of significant expense and concern.
The other basic approach to power amplifier construction is an all solid state switching design such as a class D full bridge circuit. The advantage of this design is high efficiency. The disadvantage is the difficulty in constructing an output signal having low harmonic distortion when the desired output frequency is high. This is due to the limitation in switching frequency of currently available power switches (FET's, Bipolar's, etc.). Low harmonic distortion is critical because of interference in other frequency bands.
In U.S. Pat. No. 4,523,152 there is described an invention titled: "High-Efficiency Feedforward-error-correction Amplifier". This patent describes an amplifier that uses a combination of linear and switching amplifiers in pursuit of a high efficiency and low distortion output. The waveforms combined are identical, usually nominally sinusoids. In this invention an input signal is fed to the linear amplifier with its output being fed to the switching amplifier. The switching amplifier produces a scaled or amplified replica of the linear amplifier's output. The output of the switching amplifier is fed back to the input of the linear amplifier to produce an output of this system.
This patented scheme employs switching elements within its switching amplifier whose physical limitations prohibit high power (tens of kilowatts), high frequency (&gt;50 kilo Hertz (kHz) max) operation. When pulse wave modulation is used as described in this patent, the amplifier's output frequency will become much more limited. In addition, because of the feedforward, feed-back nature of this invention, inevitable phase delays will result. Further, an optional low pass filter is proposed, but appears as an essential element to eliminate switching harmonics. This latter requirement will impede high power implementations.
There is thus a need for a high power, low distortion, high efficiency amplifier capable of a broad range of output frequencies, that is simple in operation, and that does not experience undesired phase delays.